Buggy code:
public class Env extends Iced { static final int ID = 0 ; static final int ARY = 1 ; static final int STR = 2 ; static final int NUM = 3 ; static final int FUN = 4 ; static final int SPAN = 5 ; static final int SERIES = 6 ; static final int LARY = 7 ; static final int AST = 8 ; static final int VEC = 9 ; static final int NULL = 99999 ; final transient Env . ExecStack _stack ; final IcedHashMap < Vec , IcedInt > _refcnt ; public final transient StringBuilder _sb ; final transient HashSet < Key > _locked ; final transient Env . SymbolTable _global ; final transient Env . SymbolTable _local ; final Env _parent ; private final boolean _isGlobal ; final HashSet < ValFrame > _trash ; Env ( HashSet < Key > locked ) { } Env capture ( ) { } private Env ( Env e ) { } public boolean isGlobal ( ) { } public int sp ( ) { } public void push ( Val o ) { } public void pushAry ( Frame fr ) { } public void pushHelper ( Val o , Env . SymbolTable t ) { } public boolean isEmpty ( ) { } public Val peek ( ) { } public Val peekAt ( int i ) { } public int peekType ( ) { } public Frame peekAryAt ( int i ) { } public int peekTypeAt ( int i ) { } public boolean isAry ( ) { } public boolean isNum ( ) { } public boolean isStr ( ) { } public boolean isId ( ) { } public boolean isFun ( ) { } public boolean isNul ( ) { } public boolean isSpan ( ) { } public boolean isSeries ( ) { } public Val pop ( ) { } public void pop ( int n ) { } public void poppush ( int n , Val v ) { } public Frame popAry ( ) { } public double popDbl ( ) { } public String popStr ( ) { } public ValSeries popSeries ( ) { } public ValSpan popSpan ( ) { } public Frame peekAry ( ) { } public double peekDbl ( ) { } public water . rapids . AST pop2AST ( ) { } public void toss ( ValFrame f ) { } public void clean ( ) { } private void addRef ( Val o ) { } public void addRef ( Frame f ) { } public void subRef ( Frame f ) { } private void addRef ( Vec v ) { } private void subRef ( Val o ) { } public boolean subRef ( Vec v ) { } void addKeys ( Frame fr ) { } void addVec ( Vec v ) { } static Futures removeVec ( Vec v , Futures fs ) { } void cleanup ( Frame ... frames ) { } private void extinguishCounts ( Object o ) { } private void extinguishCounts ( Vec v ) { } public void postWrite ( ) { } public void remove_and_unlock ( ) { } public void unlock ( ) { } void remove ( Object o , boolean popped ) { } void unload ( Object o , boolean popped ) { } private void subref_and_unlock ( Frame fr ) { } void popScope ( ) { } private void remove_and_unlock ( Frame fr ) { } private boolean lockerKeysNotNull ( Frame f ) { } private boolean any_locked ( Frame fr ) { } public String toString ( int i ) { } @ Override public String toString ( ) { int sp = sp ( ) ; String s = "{" ; <START_BUG> for ( int i = 0 ; i < sp ; i ++ ) <END_BUG> s += ( toString ( i ) ) + "," ; return s + "}" ; } private interface Stack { Val peek ( ) { } Val peekAt ( int i ) { } Val pop ( ) { } void push ( Val t ) { } boolean isEmpty ( ) { } int size ( ) { } int peekType ( ) { } int peekTypeAt ( int i ) { } } private class ExecStack implements Env . Stack { private final ArrayList < Val > _stack ; private int _head ; private ExecStack ( ) { } @ Override public Val peek ( ) { } @ Override public Val peekAt ( int i ) { } @ Override public int peekType ( ) { } @ Override public int peekTypeAt ( int i ) { } private int getType ( Val o ) { } @ Override public boolean isEmpty ( ) { } @ Override public int size ( ) { } @ Override public Val pop ( ) { } public void popAll ( ) { } @ Override public void push ( Val t ) { } } class SymbolTable extends Iced { HashMap < String , Frame > _frames ; HashMap < String , Env . SymbolTable . SymbolAttributes > _table ; public SymbolTable ( ) { } void clear ( ) { } public void copyOver ( Env . SymbolTable s ) { } public void put ( String name , int type , String value ) { } public int typeOf ( String name ) { } public int typeOf2 ( String name ) { } public String valueOf ( String name ) { } public void writeType ( String name , int type ) { } public void writeValue ( String name , String value ) { } private class SymbolAttributes { private int _type ; private String _value ; SymbolAttributes ( int type , String value ) { } public int typeOf ( ) { }

Fix:
for ( int i = ( - sp ) + 1 ; i <= 0 ; i ++ )